Coating powders (also known in the industry as powder coatings) are a mixture of fine particles that are utilized to provide a protective coating on an object. Such powders comprise a mixture of thermoplastic or thermosetting polymeric resin materials, pigments, cross-linking agents, catalysts, flow aids and other materials.
Currently, coating powders are generally used by applying such powders to the object being coated using electrostatic powder spray or fluidized bed coating equipment. In the electrostatic process, the coating powder is dispersed in an airstream and passed through a high voltage field where the particles pick up an electrostatic charge. The charged particles are attracted to and deposited on the object to be coated which is usually at room temperature. The object is then placed in an oven where the powder melts, cures and forms a coating on the object. In fluidized bed applications the object to be coated is heated to a temperature above the curing temperature of the coating powders. This heated object is then passed through an air fluidized cloud of the coating powders. The coating powders melt and cure as they come into contact with the hot object. Coating powders are generally utilized to produce coatings with film thicknesses of about 0.8 to 50 mils.
Coating powders are generally produced by first dry mixing the various constituents that form the powder (e.g., a base resin, a curing agent, pigments, flow agents). This mixture is then fed into a suitable melt mixer such as a twin screw extruder which melt mixes the constituents at an elevated temperature (e.g., about 250.degree.-400.degree. F.). The molten mixture which is produced by the extruder is then cooled by use of chill rollers and mechanically flaked. Finally, the flaked material is milled in an air classification mill to an average particle size of about 20-50 .mu.m.
The conventional processing of coating powders does present some downsides. For example, the high temperature processing in the extruder can bring about the premature reaction of the curing agent or the degradation of the polymer resins. Additionally, the particles produced by the conventional method tend to vary greatly in size thereby requiring the use of various particle separation techniques such as screening and/or cyclone separation in order to separate undesirable large and small particles. Particles of undesirable size must then be downgraded or disposed of. The conventional method can produce particles that do not display uniformity in composition. For example, some particles may contain excess pigment while other particles may contain too little pigment. The conventional process also presents a drawback in that there is a limit to the amount of raw materials that can be mixed at one time. More particularly, most conventional processing equipment can only handle about 2000 pounds of material during the dry mixing operation.